Recently, researches have been in progress to offer users services with a variety of service qualities, capable of transmitting high-capacity data at high speed. Especially, the researches are focusing on supporting a Broadband Wireless Access (BWA) communication system such as a Wireless Local-Area Network (WLAN) system and a Wireless Metropolitan-Area Network (WMAN) system, with high-speed service guaranteeing mobility and quality. Typical examples of the communication system include Institute of Electrical and Electronics Engineers (IEEE) 802.16a and IEEE 802.16e.
The IEEE 802.16a and the IEEE 802.16e apply Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to support a physical channel of the WMAN system with a broadband transmission network. More particularly, the IEEE 802.16a considers only a single cell structure in a state where a current Subscriber Station (hereinafter, referred to as SS) is fixed. In other words, the IEEE 802.16a does not consider mobility of the SS at all. On the other hand, the IEEE 802.16e considers mobility of the SS. The SS having mobility is referred to as a Mobile Subscriber Station (hereinafter, referred to as MSS).
Similarly, a Wireless Broadband Internet (WiBro), which is Korean standard of the wireless portable Internet, applies the OFDM for a signal transmission to enable high-speed data service in a state where the user is moving in the wireless environment. Additionally, the OFDMA based on the OFDM is applied to enable the Internet service to be offered to a plurality of users simultaneously.
In the communication system supporting the OFDM/OFDMA, change of a channel in a frequency domain may be caused by the followings. For example, a phase change of the channel transmitting signals in a frequency domain may differ depending on a position of performing Fast Fourier Transform (FFT) with a received signal in a receiver. A change of channel in the frequency domain may differ depending on a difference in time delay among the received multipath channels in a time domain.
In the communication system in which the channel is changed as described above, channel estimation is performed to cope with the change of channel. For the channel estimation, the transmitter inserts pilots in the frequency domain when performing transmission.
The receiver performs the channel estimation for compensating distortion on data between the pilots, using the pilots being transmitted. To be more specific, the channel can be estimated by averaging the pilots or by multiplying each pilot by a weight according to the position of the data and summing the multiplied values.
For example, when the pilots are relatively similar to one another in the frequency domain, noise can be reduced by the channel estimation method of the averaging the pilots. Actually, however, since the pilots are respectively different, an error in the channel estimation may be generated, when averaging the pilots.
The channel estimation method of multiplying each pilot by a weight and summing the multiplied values compensates the difference of channel by giving a greater weight to a nearer pilot. Here, when applying the same weight to all the pilots, the channel is estimated in the same manner as the method of the averaging the pilots. If different weights are given according to the pilots, adaptability for the change of channels can be improved whereas noise reduction efficiency is deteriorated.
Consequently, a channel estimation method which is not subject to the channel difference and the noise has been required.